The present invention relates to a method and system in accordance with this method for the implementation of numerical connections in particular through channels dispersive in frequency and rapidly varying in time in addition to being noisy.
The employment of radioelectrical channels for high-speed numerical connections is limited by the presence of intersymbolic interference (ISI) and propagation fading with characteristics generally varying in time and caused by the multiplicity of paths present between the transmitter and the receiver. In this situation the signal received is expressed in the equivalent model (complex) in time-discrete base band (symbol Ts time sampled), as:       y    ⁡          (      i      )        =                              ∑                      L            -            1                                    m          =          0                    ⁢                        g          ⁡                      (                          i              ;              m                        )                          ⁢                  a          ⁡                      (                          i              -              m                        )                                +          v      ⁡              (        i        )            
where a(i) is the numerical sequence transmitted (e.g. PSK, QAM, GMSK equivalent), v(i) is the added noise (thermal noise plus external interference) and g(i;m), m=0, . . . , Lxe2x88x921 are the L coefficients of the time-variant impulsive response of the transmissive channel including the transmission and reception filters.
In particular the temporal variability of the transmission channel can be induced by the mobility of the transmitter and/or receiver as happens in mobile radio connections where it increases with the speed of the vehicle or by the fluctuations in time of the altitudes of the strata of the atmosphere as is found in HF radio bridges. This variability can be expressed by the coherence time tc which is defined as the interval of time within which the channel does not vary significantly from a statistical view point or, equivalently, in terms of Doppler spread Bd=1/tc. Normalizing tc with respect to the symbol time Ts gives the number Lc=tc/Ts of consecutive symbols for which the channel can be considered nearly constant in time. In HF or mobile radio applications typical values of the product of Bdxc2x7Ts go from 10xe2x88x924 (slowly varying channel) to 10xe2x88x922 (fast channel) up to a maximum of 5xc3x9710xe2x88x923 or 10xe2x88x922 (e.g. for high-speed vehicles like fast trains or planes); contrariwise, for Bdxc2x7Ts less than 10xe2x88x924 the channel can be considered virtually static.
To obtain satisfactory performance in terms of Bit Error Rate (BER) the presence of ISI makes necessary the use of non-linear equalization techniques. Among these the Maximum Likelihood Sequence Estimation (MLSE) algorithm displays reduced computational complexity thanks, for example, to its implementation by the Viterbi algorithm (MLSE-VA) and for virtually static channels it provides broadly optimal performance since the decision on a certain symbol received takes place on the basis of the symbols either preceding or following it. The MLSE operates on the basis of a decision delay of D samples received where D is generally chosen several times the length L of the impulsive response of the channel. The efficiency of the MLSE solution in its different versions known in the technical literature is such that it is commonly employed to equalize frequency-selective channels, e.g. in GSM receivers.
When the transmission channel varies in time the MLSE must be assisted by a channel estimator which at the beginning of the transmission makes the channel estimation and then during reception of the data continuously updates the estimation which is thus supplied continuously to the MLSE equalizer. This solution is termed adaptive MLSE or AMLSE. The channel estimator typically employs the data decisions at the MLSE output and is commonly implemented by an algorithm of the gradient or a Kalman filter (see for example sec. 6.8 of the book by J. G. Proakis, Digital Communication, second edition, McGraw-Hill, 1989) which is the optimal solution. But in any case the updating takes place with a delay equal to the decision delay D.
It is known that the AMLSE equalizer loses much of its efficiency when the channel varies rapidly in time because the channel estimation is constantly delayed by D symbols with respect to the data and in this interval the channel may have varied in a not negligible manner. To obviate this problem it is possible to supply the channel estimator with low-delay xe2x80x9ctrialxe2x80x9d decisions to then make the final decisions with a greater decision delay. This mitigates but doesn""t solve the above mentioned problem because the trial decisions are less reliable than the final ones. Another possible solution is the one based on the so-called Per-Survivor Processing (PSP) which employs an MLSE equalizer in which a different channel estimation is performed for every possible surviving path. This gives significant improvements in performance at the cost however of considerable computational complexity caused by the high number of channel estimators needed.
The general purpose of the present invention is to obviate the above mentioned drawbacks by making available a method and a transmission system applying this method for transmission and reception of numerical signals which would allow high efficiency even in the case of noisy, frequency-dispersive channels rapidly variable in time.
In view of this purpose it was sought to provide in accordance with the present invention a transmission and reception method for digital signals comprising information data propagated in a communication channel in which are present the following steps. Implement a frame comprising information data sequences to be transmitted spaced by probe sequences consisting of known data, transmit the frame in the channel, receive the frame from the channel, calculate a channel characteristics estimation for probe sequences contained in the received frame, perform for data sequences an interpolation of the channel estimation found for the probe sequence, and perform acknowledgement of information data received equalizing the data sequences by the use as a channel characteristics estimation for the data sequences of the result of the interpolation.
Again in accordance with the present invention it was sought to provide a method for estimation of the characteristics of a communication channel during transmission over it of information data sequences, comprising the following steps. Intercalate with the transmitted data sequences probe sequences consisting of known data, calculate the channel characteristics estimation for probe sequences, interpolate the calculated estimation, and employ the interpolation result as a channel characteristics estimation during reception of information data sequences.
It was also sought to provide an apparatus for transmission and reception of digital signals comprising information data propagated in a communication channel characterized in that it comprises a transmitting apparatus and a receiving apparatus at the two ends of the channel with the transmitting apparatus comprising in turn an intercalating device receiving at input the information data to be transmitted and outputing a frame made up of information data sequences to be transmitted intercalated with probe sequences consisting of known data and a transmitter for sending the frame over the channel with the receiving apparatus comprising in turn a receiver for reception of the frame from the channel with a discriminator separating the probe sequences and the data sequences from the received frame, an estimator receiving at input the probe sequences and supplying at output the channel estimation characteristics for the received probe sequences, an interpolator receiving at input the estimation output by the estimator and producing by interpolation the desired channel characteristics estimation for the data sequences and an equalizer receiving the estimation output by the interpolator and equalizing on the basis thereof the data sequence extracted from the discriminator to output the correct sequence of transmitted data.